documentation: atomic_add_unless() doesn't imply mb() on failure

(sorry for being offtpoic, but while experts are here...)

A "typical" implementation of atomic_add_unless() can return 0 immediately
after the first atomic_read() (before doing cmpxchg). In that case it doesn't
provide any barrier semantics. See include/asm-ia64/atomic.h as an example.

We should either change the implementation, or fix the docs.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 3 insertions, 2 deletions

diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index f20c10c2858f..4ef245010457 100644
--- a/Documentation/atomic_ops.txt
+++ b/Documentation/atomic_ops.txt
@@ -186,7 +186,8 @@ If the atomic value v is not equal to u, this function adds a to v, and
 returns non zero. If v is equal to u then it returns zero. This is done as
 an atomic operation.
 
-atomic_add_unless requires explicit memory barriers around the operation.
+atomic_add_unless requires explicit memory barriers around the operation
+unless it fails (returns 0).
 
 atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
 
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 4e17beba2379..1f506f7830ec 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1493,7 +1493,7 @@ explicit lock operations, described later).  These include:
         atomic_dec_and_test();
         atomic_sub_and_test();
         atomic_add_negative();
-        atomic_add_unless();
+        atomic_add_unless();    /* when succeeds (returns 1) */
         test_and_set_bit();
         test_and_clear_bit();
         test_and_change_bit();